Courtesy: Nicole Kistler

Seattle is building one of the country's biggest rooftop planting spaces.

When the organizers of the 1962 Seattle World’s Fair imagined the future, they probably didn’t envision, among the jet packs and routine space travel, tomatoes growing on the roof of a parking garage.

But 50 years later, that’s exactly what’s about to happen a few blocks from the Space Needle, where residents are building a 30,000-square-foot community garden atop a two-story structure once intended for fair visitors’ cars.

"As far as we can tell it’s the first community-managed food production garden on a rooftop” in the country, says Eric Higbee, a landscape architect working on the project. This project, dubbed the UpGarden, will have space for about 120 gardeners. There are a few rooftop farms, such as Eagle Street Rooftop Farm in Brooklyn. But a commercial operation like that runs around $10 per square foot to construct, while the UpGarden has shoestring budget of $4 per square foot—and it’s designed to be built and maintained almost entirely by volunteers.

The project came about because Seattle’s P-Patch community gardening program was looking for space to build a new garden in the neighborhood. “We were really struggling, because the neighborhood is really dense,” says P-Patch coordinator Laura Raymond.

But building a rooftop garden isn’t straightforward. "You’d think that cars are really heavy, and you could put anything on top of a garage," says Nicole Kistler, a landscape designer and artist also on the design team. In fact, soil is much heavier—12 inches of water-saturated soil can weigh over 100 pounds per square foot, but the garage is only designed to support 40 pounds per square foot.

“We had to find a way to get enough soil up there to grow vegetables, but also not exceed the weight capacity of the garage,” Higbee says. "That really drove a lot of the design decisions.”

Typical green roof technologies were too expensive, so they settled on a series of wooden raised beds 12 to 18 inches deep, which will be filled with potting soil. It’s lighter than topsoil. Higbee and Kistler also left wide paths between the garden beds.

At $150,000, designing and building the UpGarden will cost about 10 percent more than a ground-level community garden of similar size, Raymond estimates. The increased costs come mainly from a longer, more elaborate design process, the need for a structural engineer, and a contractor to drill into the garage deck. In addition, the low clearance of the garage means that materials like potting soil and wood chips will have to be blown in, rather than a large load being dumped by a truck and wheelbarrowed into place by volunteers.

Courtesy: Nicole Kistler

An unexpected splurge was craning in the vintage Airstream trailer acquired on the cheap that will serve as a toolshed. The decorative 1963 Ford Galaxy painted iridescent purple was procured entirely free of charge. The interior has been stripped out, and the group envisions transforming this “relic of car culture,” as they call it, into a kind of glasshouse for planting pumpkins and corn. (The only downside: squash vines could obscure the Jimi Hendrix quotes painted on the windows.)

Learning to grow these and other vegetables successfully on a rooftop will be the next challenge. On the plus side, roofs are bright and hot, which should make it easier to grow hot-weather crops like tomatoes, peppers, and eggplant. But rooftops are also windy and the soil will dry out faster, a problem magnified by the lightweight soil and shallow garden beds. To prevent that fate, the team is considering a drip irrigation system.

Whatever the group decides on, the setup is likely to be a temporary one: the Mercer Garage is slated to be torn down in 3 to 5 years, and the garden will have to move. But the knowledge developed in the meantime may aid in the development of other rooftop gardens in Seattle and elsewhere.

Already, Raymond has been contacted by a group in the nearby city of Bellingham planning a similar project. And familiarity with rooftop weight limits helped P-Patch quickly pull together a smaller rooftop garden for Chancery Place Apartments, a low-income senior housing facility, earlier this spring.

“Rooftop gardens are complicated, and they aren’t the right fit for all places,” Raymond says. “But I think in areas where there are rooftops that can support a garden, and there isn’t land at ground level, then it makes a lot of sense.”

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